Russian 'Spetsnaz' special forces troops have surrounded the Syrian city of Aleppo amid fierce fighting in the besieged city, according to reports, Express reported.

Commandos sent by the Kremlin are apparently working covertly to support Syrian government forces who say they are close to retaking parts of the city from ISIS control.

Russian airstrikes have intensified in recent days in a bid to crush ISIS and rival rebel groups that have laid claim to vast swathes of the city since the civil war erupted in 2011.

David Daoud, a Middle East analyst at the Foundation for Defence of Democracies, told an American news website: "Russian-Hezbollah cooperation and coordination in Syria has been going on since the outset of Russia's entry into the civil war."

"So it wouldn't be surprising that there's now cooperation between Hezbollah's ground forces and Russian special forces."

The last two days has seen government troops alongside Lebanese fighters encircle the countryside north of Aleppo and cut off the main supply route linking the city - Syria's largest before the war - to Turkey.

Ankara said it suspected the aim was to starve the population into submission.

Tens of thousands of Syrians have fled the area.

Video footage showed thousands of people massing at the Bab al-Salam crossing on the Turkish border.

Turkish Prime Minister Ahmet Davutoglu said on Friday 15,000 people had arrived at the Turkish border. while a Turkish charity says about 50,000 people have amassed at the border crossing.

"The regime is now trying to expand the area it has taken control of.

"Now the northern countryside (of Aleppo) is totally encircled, and the humanitarian situation is very difficult."

Reclaiming Aleppo would be the biggest strategic prize in years for President Bashar al-Assad's government - and would represent a turning point in the five year war.